Multi-frequency signal receivers are frequently used in telephony systems, particularly with multi-frequency register signalling for the detection of both analog signals and digital signals, which signals may be derived from analog signals, possibly by sampling and analog-to-digital conversion. Such a multi-frequency signal receiver arranged for the detection of digital signals is disclosed in the article "Digital MF Receiver Using Discrete Fourier Transform" by I-Koval and G. Gara, published in IEEE Transactions on Communications, vol. COM-21 No. 12, December 1973, pages 1331-1335.
The frequencies of the said signalling type are located within the audio band. Thus, it is possible that interfering signals located in the audio band, such as noise and other unwanted signals, simulate signal combinations. So as to distinguish between the reception of a valid signal combination or the reception of interfering signals these receivers are provided with an interfering signal guard circuit.
Thus, the interfering signal guard circuit in the multi-frequency signal receiver disclosed in the above-mentioned article determines the total received power and compares this with the power of the two strongest received signals in order to determine whether the detected signal combination is valid.
On the occurence of an interfering signal consisting of a single frequency signal, whose frequency is located approximately halfway between two consecutive signal frequencies the two frequency-selective elements which are sensitive to the two signals will respond and the interfering signal guard circuit will not detect any other power than the power detected in the said two frequency-selective elements. Consequently, this multi-frequency signal receiver will not recognize as a valid signal combination a signal combination simulated by a single frequency signal.
To obviate this it is known from French Patent Specification No. 2,455,405 to use comb filters whose zero points in their transfer characteristics substantially coincide with the nominal frequencies of the group of multi-frequency signals. As the addition of a comb filter function increases the detection period as the processing period is increased, or additional or faster components are required, it is known from the French Patent Specification to calculate the comb filter function simultaneously with the signal frequencies by utilizing the side lobes of the filter transfer characteristic. This results however in a coupling between the audio signal detection function and a single frequency interfering signal detection.